DE-A-37 26 643 describes a process in which brown coal having a considerable water content is intensely mixed with a hot combustion residue in a fluidized bed and the mixture is fed to a fluidized bed combustion chamber and is combusted therein. The flue gas rising from the combustion chamber entrains combustion residue and is passed through a cyclone and the residue is separated in the cyclone and is then fed to the fluidized bed and is mixed therein with the water-containing brown coal.
In that known process, gaseous products of carbonization, particularly carbon dioxide, methane and hydrogen, are withdrawn at certain rates together with vapor from the fluidized bed which serves to dry the brown coal. The products of carbonization are formed when previously dried brown coal is overheated in the fluidized bed. Such overheating cannot entirely be avoided because the fluidized bed essentially consists of a mixture of previously dried coal particles and cooled combustion residue. For this reason the hot residue will always be contacted with previously dried brown coal and that contact will result in a local overheating and carbonization. The products of carbonization in the water vapor involve in the first place a decrease of the heating value of the fuel mixture which is supplied to the fluidized-bed combustion chamber and in the second place are disturbing in the utilization of the water vapor which has been formed by the drying.
In the process known from DE-A-37 26 643, fluidizing steam is required at a considerable rate and that fluidizing steam must subsequently be dedusted together with the evaporated water and must be recycled to the fluidized bed by a compressor.